A gold signal delivers a trade idea with entry, invalidation and target and leaves execution and risk management with the trader, while a gold expert advisor executes automatically on the account and removes discretion along with the trader's awareness of what conditions the logic was tested against. Signals tend to fail slowly and visibly through slippage and chased entries; expert advisors tend to fail suddenly through regime change and cost sensitivity.
The failure mode of most gold expert advisors is curve fitting to a single volatility regime. Logic that appeared stable across months of a quiet range can surrender its entire gain in one session when gold's daily range doubles during a macro repricing, or when spread widens from twenty cents to two dollars through an inflation release, which is fatal to grid, martingale and high frequency structures.
The only defensible comparison is on verifiability, cost sensitivity and failure mode rather than marketing. Any service or system that cannot show a complete record including losing trades on a disclosed sample size is not assessable at any price, and either route should be journaled trade by trade so realised results can be compared against the published record before capital is committed.